Ongoing specification work by the Third Generation Partnership Project, 3GPP, includes, among other things, a focus on improving support for heterogeneous network operations and other multi-point scenarios, such as Coordinated Multi-Point, CoMP networks. As will be appreciated by those skilled in the art, Long Term Evolution, LTE, networks use Orthogonal Frequency Division Multiplexing, OFDM, in the downlink and Discrete Fourier Transform, DFT, spread OFDM in the uplink.
An OFDM carrier comprises a plurality of spaced-apart narrowband carriers, often referred to as subcarriers, each of which can be individually modulated in each OFDM symbol interval. Each subcarrier within a given OFDM symbol time thus represents a “resource element” and the overall OFDM carrier thus can be seen as a time-frequency grid of individual resource elements. More particularly, each resource element corresponds to one subcarrier during one OFDM symbol interval on a particular “antenna port” of a transmitting base station. Here, an antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. There is one OFDM resource grid per antenna port.
LTE “broadcasts” control messages to user equipments, UEs, using the Physical Downlink Control Channel or PDCCH. PDCCH transmissions occur in the first one to four OFDM symbol intervals in each OFDM subframe, where there are ten OFDM subframes per OFDM frame. OFDM frames are ten milliseconds long, hence each subframe spans one millisecond and includes two slots of one-half millisecond per slot. In LTE, the subframe represents one Transmission Time Interval or TTI, and is the unit of scheduling. Cell specific Reference Signals or CRS are transmitted across the OFDM bandwidth and allow UEs to perform channel estimation for PDCCH reception, regardless of their location relative to the antenna port(s) used for PDCCH/CRS transmissions.
More recently, LTE has been extended to include “enhanced” control channels, such as the enhanced PDCCH, which is referred to as the EPDCCH or the ePDCCH. These enhanced control channels are UE-specific and demodulation of them at the targeted UEs relies on the use of UE-specific Demodulation Reference Symbols or DMRS, which are transmitted from the same antenna port, or, more generally, the same transmission point as used for transmitting the ePDCCH for a given UE. By transmitting DMRS from the individual transmission points used for transmitting user-specific control information, the targeted UEs are able to estimate the channel with respect to those individual transmission points. DMRS also serve the same purpose for multi-point data transmissions, such as used in CoMP.
In more detail, CoMP transmission techniques introduce dependencies in the scheduling or transmission/reception among different transmission points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points. DL CoMP operations may include, e.g., serving a certain UE from multiple points, either at different time instances or for a given subframe, on overlapping or not overlapping parts of the spectrum. Dynamic switching between transmission points serving a certain UE is often termed as DPS (dynamic point selection). Simultaneously serving a UE from multiple points on overlapping resources is often termed as JT (joint transmission). The point selection may be based, e.g., on instantaneous conditions of the channels, interference or traffic. CoMP operations are intended, for example, for data channel transmissions, such as on the Physical Downlink Shared Channel, PDSCH, used in LTE and/or control channel transmissions, such user-specific ePDCCH transmissions in LTE.
The introduction of user-specific DMRS complicates channel estimation and corresponding signal reception in, the UEs, as individual UEs must perform channel estimation using a potentially smaller set of DMRS, which by their nature are not as dense or numerous within the OFDM grid as CRS or other common reference signals. As a further complication, further revisions of LTE or other evolving network standards may introduce new carrier types, which may not carry all or the same reference signals as the currently defined carrier types. For example, a new carrier type may replace the CRS with a “reduced” CRS port that is transmitted with reduced periodicity in time and with a reduced bandwidth. The bandwidth of the reduced CRS port may also be configurable. In another example, a new carrier is transmitted in synchronization with a legacy carrier, in a carrier-aggregated mode. In this synchronized carrier case, the new carrier may not transmit the Primary and Secondary Synchronization Signals, PSS and CSS, and/or the CRS, which are provided in the legacy carrier for use by UEs in synchronizing with the legacy carrier and making corresponding channel estimates.
Broadly, whether in CoMP or other network arrangements, different reference signals may be transmitted from different antenna ports and those ports may be geographically separated. Geographical separation of ports implies that instantaneous channel coefficients from each port towards a given wireless device are in general different. Furthermore, even the statistical properties of the channels for different antenna ports and different reference signal types may be significantly different. Examples of such statistical properties include the received power for each port, the delay spread, the Doppler spread, the received timing (i.e., the timing of the first significant channel tap), the number of significant channel taps, the frequency shift.
As a general proposition in LTE, for example, a wireless device cannot make any assumptions about the properties of the channel corresponding to a given antenna port based on the properties of the channel of another antenna port. This independence is in fact a key part of maintaining transmission transparency with respect to the wireless devices operating within an LTE network. The wireless device, in general, thus needs to perform an independent estimation for each reference signal port of interest, for a given transmission. But this arrangement can result in occasionally inadequate channel estimation quality for certain reference signal ports, and that inadequacy may lead to undesirable link and system performance degradation. For example, the network in question may configure a wireless device to use CRS, DMRS, and/or Channel State Information Reference Signals, CSI-RS, for estimation of propagation parameters and/or for generating link adaptation feedback, to aid the reception of other signals.